X-ray imaging system

ABSTRACT

An X-ray imaging system has an X-ray source, an X-ray image detection section, an X-ray mask member, a drive unit, a memory, a calculating section, and an image output unit. An X-ray image is detected by the X-ray image detection section. The X-ray mask member has X-ray shielding regions, distributed in a predetermined pattern, which locally shield the X-ray. The mask member is driven by the drive unit so that it is inserted in or removed from an X-ray radiation field between the X-ray image detection section and the X-ray source, and is sequentially displaced to predetermined positions in the radiation field. The calculating section calculates scattered X-ray intensity distribution data based on a plurality of transmission X-ray data obtained by irradiating an object with X-rays with the mask member located at different positions in the radiation field, and transmission X-ray data obtained by irradiating the object with X-rays with the mask member located outside this field. The calculating section then calculates X-ray image data from the scattered X-ray intensity distribution data and transmission X-ray data obtained with the mask member located outside the radiation field.

BACKGROUND OF THE INVENTION

The present invention relates to an X-ray imaging system whichirradiates an object with X-rays, and detects X-rays transmitted throughthe object to obtain transmission X-ray data, thereby forming a visibleimage based on the transmission X-ray data and, more particularly, to ascattered X-ray elimination technique suitable for a so-called digitalradiography apparatus which converts an X-ray image into digital data.

In conventional X-ray imaging systems, a detector for detecting X-raysreceives direct X-rays transmitted through an object without beingscattered, as well as scattered X-rays scattered by the object. Thescattered X-rays are a major factor contributing to the degradation ofcontrast and sharpness of an X-ray image obtained through the detector.For this reason, in X-ray imaging systems, it is very important toeliminate scattered X-rays.

In order to eliminate scattered X-rays, a grid is usually used inconventional systems. However, since the grid itself generates scatteredX-rays, it cannot perform satisfactory elimination of scattered X-rays.

If scattered X-rays can be eliminated, a contrast and sharpness of anX-ray image can be improved, thus providing a good X-ray image. Inaddition, if an image based on direct X-rays can be obtained,attenuation of the X-rays by the object can be accurately calculated bylogarithmic conversion of the image data. Therefore, it is verydesirable to eliminate scattered X-rays.

Although various studies have been made on the nature of these scatteredX-rays, since X-ray scattering involves complicated phenomena, manyaspects thereof still remain unsolved.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an X-ray imagingsystem which effectively eliminates scattered X-ray components containedin image data, and which can form an X-ray image of high contrast andsharpness without blurring.

In order to achieve the above object of the present invention, there isprovided an X-ray imaging system comprising: an X-ray source foremitting X-rays to be radiated on an object; an X-ray image detectionsection for detecting an X-ray image emitted from the X-ray source andtransmitted through the object; an X-ray mask member, having a pluralityof X-ray shielding regions distributed in a predetermined pattern, forlocally shielding X-rays with the plurality of X-ray shielding regions;a drive unit for moving the X-ray mask member so that the X-ray maskmember is inserted or removed with respect to an X-ray radiation fieldbetween the X-ray image detection section and the X-ray source and issequentially positioned at a plurality of predetermined positions in theX-ray radiation field; a storage section for storing X-ray image data; afirst calculating section, associated with the storage section, forcalculating scattered X-ray intensity distribution data associated withthe object, based on a plurality of transmission X-ray data obtained byirradiating the object with X-rays when the X-ray mask member is locatedat different positions in the X-ray radiation field, and on transmissionX-ray data obtained by irradiating the object with X-rays when the X-raymask member is located outside the X-ray radiation field; a secondcalculating section, associated with the storage section, forcalculating X-ray image data, from which the influence of scatteredX-rays is eliminated, in accordance with the scattered X-ray intensitydistribution data obtained by the first calculating section andtransmission X-ray data obtained by irradiating the object with X-rayswhen the X-ray mask member is located outside the X-ray radiation field;and an image output unit for outputting the X-ray image data calculatedby the second calculating section as a visible image.

The present invention provides an X-ray imaging system, which cansufficiently eliminate scattered X-ray components contained in the imagedata and can form an X-ray image of high contrast and sharpness withoutblurring. More specifically, in the X-ray imaging system of thisinvention, a plurality of masked X-ray data, produced by an X-ray maskmember displaced to different positions in an X-ray radiation field, aresubjected to calculation of scattered X-ray intensity distribution, thusgreatly improving calculation precision of the scattered X-raycomponents contained in the image data. Therefore, the scattered X-raycomponents contained in the image data can be effectively eliminated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an X-ray imaging system according to anembodiment of the present invention;

FIG. 2 is a schematic plan view of an X-ray mask member used in thesystem in FIG. 1;

FIG. 3 is an illustration for explaining an operation principle of thesystem in FIG. 1;

FIG. 4 is a graph showing an X-ray intensity distribution detected whenX-rays are radiated in a state shown in FIG. 3;

FIGS. 5A to 5C and 6 are illustrations for explaining movement of theX-ray mask member of the system in FIG. 1 in an X-ray radiation field;

FIGS. 7A to 7C are timing charts respectively showing an X-ray radiationtiming, a readout timing of an X-ray image from a camera, and anoperation timing of the X-ray mask member in an X-ray imaging system towhich the present invention is applied; and

FIGS. 8A to 8C are illustrations for explaining operations of thebilevel quantization.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As shown in the block diagram of FIG. 1, an X-ray imaging systemaccording to an embodiment of the present invention comprises X-ray tube1, X-ray generation unit 2, collimator 3, X-ray mask member 4, driveunit 5, X-ray image detector 6, camera 7, A/D (analog-to-digital)converter 8, memories 9 to 14, calculating sections 15 and 16,calculating/controlling unit 17, D/A (digital-to-analog) converter 18,display 19, and data bus 20.

When X-ray tube 1 is driven by X-ray generation unit 2, X-rays areemitted from tube 1 toward object (e.g., a patient) P. A radiation fieldof X-rays emitted from tube 1 is restricted by collimator 3. The X-rays,the radiation field of which is defined by collimator 3, become incidenton object P through X-ray mask member 4, which partially shields X-rays.

Mask member 4 comprises X-ray transmitting plate 31 of, e.g., a thinacrylic resin plate having good X-ray transmittance, on which aplurality of small segments (e.g., lead segments) 32 of an X-rayshielding material are attached at equal intervals, as shown in FIG. 2.Each lead segments 32 has a size of 2 mm×2 mm, for example.

Mask member 4 locally shields the X-rays with a predetermined patternconstituted by the plurality of lead segments 32. Mask member 4 is movedby drive unit 5 to be inserted in or removed from the X-ray radiationfield. In addition, when member 4 is located in the X-ray radiationfield, it can be displaced to different positions therein upon operationof drive unit 5.

X-ray image detector 6 comprises, for example, an image intensifier(I.I.), and detects X-ray image data transmitted through object P toconvert it into a visible image. The visible X-ray image obtained bydetector 6 is converted into an electrical signal by camera (e.g., TVcamera) 7, and is then converted into a digital signal by A/D converter8.

1st memory 9 stores transmission X-ray data (original image data XO)transmitted through object P when mask member 4 is located outside theX-ray radiation field. 2nd memory 10 stores X-ray image data (firstmasked X-ray data MA) obtained when mask member 4 is located at a firstpredetermined position in the X-ray radiation field. 3rd memory 11stores X-ray data (second masked X-ray data MB) obtained when maskmember 4 is located at a second predetermined position in the X-rayradiation field separated from the first predetermined position.

1st calculating section 15 performs subtraction processing of originalimage data XO stored in 1st memory 9 and first masked X-ray data MAstored in 2nd memory 10 for each corresponding pair of pixels, andstores resultant first subtraction data SA in 4th memory 12. Calculatingsection 15 also performs subtraction processing of original image dataXO and second masked X-ray data MB stored in 3rd memory 11 for eachcorresponding pair of pixels, and stores resultant second subtractiondata SB also in 4th memory 12. Calculating section 15 also performssubtraction processing of original image data XO and scattered X-raydistribution data DS (to be described later) indicating a scatteredX-ray intensity distribution for each corresponding pair of pixels, andoutputs the processing result to D/A converter 18.

Calculating/controlling unit 17 comprising, e.g., a CPU (centralprocessing unit), controls the operation of the entire system (e.g., theoperation of X-ray generation unit 2 for driving X-ray tube 1, driveunit 5 for driving mask member 4, and camera 7, read/write control ofall the memories, and the like). Unit 17 calculates a central addressfor each region shielded with lead segments 32 of mask member 4 (i.e.,an X-ray shielding region) in a memory, displacement of the address dueto movement of mask member 4, and an average value of X-ray intensitydata in the X-ray shielding regions, i.e., scattered X-ray componentdata.

5th and 6th memories 13 and 14 store transient calculation data fromunit 17. 2nd calculating section 16 performs data interpolation using,e.g., a SYNC function, on the basis of data associated with the X-rayshielding regions calculated by unit 17, thus obtaining scattered X-raydistribution data. Various data communication between respectivememories and calculating sections is made through data bus 20.

The operation of the system with the above arrangement will now bedescribed.

At the beginning of an imaging operation, mask member 4 is locatedoutside the X-ray radiation field. In this state, unit 17 causes X-raygeneration unit 2 to radiate X-rays from X-ray tube 1 toward object P.In synchronism with X-ray radiation, unit 17 controls camera 7 to obtainoriginal image data XO through X-ray image detector 6. Acquired originalimage data XO is converted into digital data by A/D converter 8, and iswritten in 1st memory 9.

Next, drive unit 5 is driven under the control of unit 17, so that maskmember 4 is moved to a first position in the X-ray radiation field. Inthis state, X-rays are emitted from X-ray tube 1, and first masked X-raydata MA is acquired by camera 7 through detector 6. Data MA is thenwritten in 2nd memory 10 through A/D converter 8.

The position of mask member 4 is shifted in the same plane to a secondposition in the X-ray radiation field slightly shifted from the firstposition. Upon re-radiation of X-rays in this state, second masked X-raydata MB is acquired by camera 7. Data MB is then written in 3rd memory11 through A/D converter 8.

The relationship between X-ray data MA and MB, written in 2nd and 3rdmemories 10 and 11, and mask member 4 will now be described in detailwith reference to FIGS. 3 to 6.

FIG. 3 is an illustration for explaining acquisition of masked X-raydata, and FIG. 4 is a graph showing an X-ray intensity distributiondetected when X-rays are radiated in the state shown in FIG. 3. WhenX-rays XR are radiated in the state wherein mask member 4 is positionedin the X-ray radiation field, an X-ray intensity distribution of maskedX-ray data along line A--A' detected by detector 7 is as shown in FIG.4. The X-ray intensity distribution in FIG. 4 shows steep dips atpositions indicated by a', b', c', d', and e'.

These dips represent that X-ray XR is locally shielded by lead segments32 (i.e., segments 32a, 32b, 32c, 32d, and 32e) along line A--A'.Although X-ray XR is shielded by lead segments 32a, 32b, 32c, 32d, and32e, values at distal ends of the dips are not zero because scatteredX-rays are detected. Thus, level and distribution of the scattered X-raycomponents can be calculated from the values of the dips. In thiscalculation, as the number of dips, i.e., the number of lead segments 32provided in mask member 4, increases, detection precision of thescattered X-ray components can be improved. However, if the number ofdips, i.e., the number of lead segments 32, is too large, the amount ofX-rays shielded by segments 32 increases, and the amount of X-raystransmitted through object P and landing on detector 6 thereforedecreases, resulting in a smaller amount of scattered X-ray componentdata than actually exists. In theory, as the number of X-ray shieldingregions included in an imaging region increases, more effectivescattered X-ray component data could be obtained. However, in practice,a distribution density of the X-ray shielding regions must be set belowa given level. Alternatively, the size of segments 32 can be reduced.However, in order to allow detection of the dips, segments 32 cannot besmaller than a given size.

Because, as seen from FIG. 8B, in general, the waveform of the X-rayintensity signal is distorted and stretched. It is therefore necessaryto distinguish the desirable signal belonging to the X-ray shieldedportion (i.e., the scattered X-ray signal) from the signal belonging tothe other portion (i.e., the primary X-ray signal and a part of thescattered X-ray signal). Therefore, as seen FIG. 8A to 8C, the X-rayintensity data of the portion which is not shielded by the lead piece 32has a higher level than the threshold level and is converted into thedigital "0" level signal. The X-ray intensity data of the portion whichis shielded by the lead piece 32 has a lower level than the thresholdlevel and is converted into the digital "1" level signal. This bilevelquantization is carried out in 1st calculating section 15.

In the system of this embodiment, instead of increasing the number ofsegments 32, X-ray mask member 4 is displaced to, e.g., two differentpositions in the X-ray radiation field, and X-ray data MA and MB areobtained during respective X-ray radiations at the two positions,thereby providing the same effect as when the number of segments 32 isincreased.

FIGS. 5A to 5C and FIG. 6 are illustrations for explaining movement ofmask member 4 in the X-ray radiation field in the system of thisembodiment. If mask member 4 in a state shown in FIG. 5A isparallel-moved by distance dx in the X direction, a state shown in FIG.5B is obtained. Assuming that the coordinates of lead segment 32p inFIG. 5A are (x, y), those in FIG. 5B are (x+dx, y). If mask member 4 inthe state in FIG. 5A is parallel-moved by distance dy in the Ydirection, a state shown in FIG. 5C is obtained, and the coordinates ofsegment 32p are (x, y+dy). X- and Y-movements can be freely combined.FIG. 6 illustrates the position of member 4 before and after it is movedin the X and Y directions by distances dx and dy, respectively, whenthese movements overlap each other. Referring to FIG. 6, after the twoparallel-movements, the coordinates (x, y) of segment 32p before themovements are changed to be (x+dx, y+dy). When member 4 is moved bydistance DX in the X direction, the coordinates of segment 32p are thesame as they were before the movements. Therefore, the amount ofmovement in the X direction can be smaller than distance DX and that inthe Y direction can be smaller than distance DY, so that the coordinatesof segment 32p are the same as those of lead segment 32r before themovements. Within this limited range, segment 32p can be moved to anylocation encompassed by segments 32p, 32q, 32r, and 32s, and the objectof this embodiment can be satisfactorily achieved thereby.

Masked X-ray data MA and MB acquired by displacing mask member 4 todifferent positions as above are subjected to subtraction with originalimage data XO.

1st calculating section 15 executes subtraction processing of originalimage data XO stored in memory 9 and X-ray data MA stored in memory 10for each corresponding pair of pixels. The subtraction result (i.e.,subtraction data SA) from section 15 is then stored in memory 12.

Calculating/controlling unit 17 calculates central addresses of theX-ray shielding regions in X-ray data MA, and scattered X-ray componentdata corresponding to the respective X-ray shielding regions inaccordance with subtraction data SA stored in memory 12. The data isthen written in memory 13. This data will be referred to as X-rayshielding region data AA hereinafter.

Calculating section 15 also executes subtraction processing of originalimage data XO and X-ray data MB stored in memory 11. The subtractionresult (i.e., subtraction data SB) from section 15 is also stored inmemory 12.

Unit 17 calculates central addresses of the respective X-ray shieldingregions in data MB, and scattered X-ray component data corresponding tothe respective X-ray shielding regions in accordance with subtractiondata SB stored in memory 12. The data from unit 17 is then stored inmemory 14. This data will be referred to as X-ray shielding region dataAB hereinafter.

Unit 17 calculates a distance between the central addresses of the X-rayshielding regions (i.e., displacement of member 4) based on data AA andAB respectively stored in memories 13 and 14, and outputs the result to2nd calculating section 16 together with data AA and AB.

Calculating unit 16 executes data interpolation using a SINC functionbased on the output data from unit 17, thus obtaining scattered X-raydistribution data indicating the scattered X-ray intensity distribution.The scattered X-ray distribution data is then supplied to section 15 tobe subjected to subtraction processing with original image data XOstored in memory 9. After the subtraction processing, the scatteredX-ray components contained in original image data XO can be eliminated,and image data corresponding to an image formed only by direct X-raycomponents can be obtained. The image data is then supplied to display19 through D/A converter 18, to be displayed as an image. Since theimage displayed on display 19 is free from the influence of scatteredX-rays, it has high contrast and sharpness.

Note that the scattered X-ray distribution data is subtracted fromoriginal X-ray data XO stored in memory 9 by section 15. Alternatively,after acquiring the masked X-ray data, X-ray radiation can be performedwhile mask member 4 is located outside the X-ray radiation field, so asto obtain transmission X-ray data. The transmission X-ray data is usedinstead of original image data XO, and the above subtraction can beperformed. Once the scattered X-ray distribution data is obtained, itcan be used to eliminate the scattered X-ray components fromtransmission X-ray data during the subsequent X-ray radiation as long asthe position or size of the X-ray radiation field or X-ray radiationconditions remain substantially the same.

FIGS. 7A to 7C respectively show timings of X-ray radiation, imagedetection of detector 6, and movement of mask member 4. In thisembodiment, camera 7 comprises a vidicon TV camera having anaccumulation type target. X-ray tube 1 radiates X-ray pulses at equaltime intervals, as shown in FIG. 7A. A transmission X-ray imageaccumulated on the target of camera 7 through detector 6 during theX-ray radiation is read out during an interval between two X-rayradiations, as shown in FIG. 7B. Mask member 4 is moved during such aninterval, i.e., during the readout interval of camera 7. In this case,when member 4 is located outside the radiation field, a first X-rayradiation is performed to acquire original image data XO. During thereadout interval of data XO from camera 7, mask member 4 is moved to afirst position in the radiation field. After this movement, a secondX-ray radiation is performed to acquire masked X-ray data MA. During thereadout interval of data MA from camera 7, mask member 4 is moved to asecond position in the radiation field. After completion of themovement, a third X-ray radiation is performed to acquire masked X-raydata MB. During the readout interval of data MB from camera 7, maskmember 4 is moved outside the radiation field. The scattered X-raydistribution data is calculated after the above sequence. TransmissionX-ray data obtained from repetitive X-ray radiation can be correctedwith the same scattered X-ray distribution data unless a position ordirection of the object, a position or size of the radiation field, andX-ray radiation conditions are greatly changed.

Masked X-ray data MA and MB need only provide positions and values ofdips caused by the X-ray shielding regions, and the number of X-rayshielding regions of member 4 can be reduced because member 4 itself isdisplaced. Even if data MA and MB, subtraction data SA and SB, and X-rayshielding region data AA and AB corresponding thereto express 1-frameimage data in a smaller (rougher) matrix size than that of originalimage data, detection precision will substantially not be degraded.Therefore, the capacities, per frame of an image, of memories 10 to 14for storing data MA, MB, SA, SB, AA, and AB can be smaller than that ofmemory 9. If original image data is constituted by 512×512 pixels, dataMA, MB, SA, SB, AA, and AB can often be expressed by 128×128 or 256×256pixels. In this case, the correspondence between pixels of therespective image data must be corrected during the above calculations.

With the system of this embodiment, scattered X-ray distribution data isobtained based on X-ray shielding data acquired when mask member 4 islocated in the X-ray radiation field, and scattered X-ray componentscontained in the original image data are eliminated through subtractionprocessing of the scattered X-ray distribution data and the originalimage data, thus displaying an image of high contrast and sharpness. Inparticular, the system can provide the same effect as when the number oflead segments 32 is increased, since a plurality of masked X-ray dataacquired when mask member 4 is at different positions in the X-rayradiation field are combined. Therefore, measurement precision of thescattered X-ray component distribution can be improved withoutincreasing the number of segments 32.

The present invention has been described in connection with a particularembodiment. However, the present invention is not limited to the aboveembodiment, and various changes and modifications may be made within thespirit and scope of the invention.

For example, in the above embodiment, movement of mask member 4 (i.e.,displacement thereof into, from, or within the X-ray radiation field) isperformed through drive unit 5 under the control of unit 17. However,mask member 4 can be moved manually.

In the system of the above embodiment, original image data is acquiredwhen mask member 4 is out of the X-ray radiation field, and X-rayshielding region data is then acquired when it is inside the field.However, the X-ray shielding region data can be obtained first, andthereafter, the original image data can be obtained.

What is claimed is:
 1. An X-ray imaging system comprising:an X-ray source for emitting X-rays to be radiated on an object; X-ray image detection means for detecting an X-ray image emitted from said X-ray source and transmitted through said object; X-ray mask means, having a plurality of X-ray shielding regions distributed in a predetermined pattern, for locally shielding X-rays with said plurality of X-ray shielding regions; drive means for moving said X-ray mask means so that said X-ray mask means is inserted or removed with respect to an X-ray radiation field between said X-ray image detection means and said X-ray source and is sequentially positioned at a plurality of predetermined positions in said X-ray radiation field; storage means for storing X-ray image data; first calculating means, associated with said storage means, for calculating scattered X-ray intensity distribution data associated with said object based on a plurality of transmission X-ray data obtained by irradiating said object with X-rays when said X-ray mask means is located at different positions in said X-ray radiation field, and on transmission X-ray data obtained by irradiating said object with X-rays when said X-ray mask means is located outside said X-ray radiation field; second calculating means, associated with said storage means, for calculating X-ray image data, from which the influence of scattered X-rays is eliminated, in accordance with the scattered X-ray intensity distribution data obtained by said first calculating means and transmission X-ray data obtained by irradiating said object with X-rays when said X-ray mask means is located outside said X-ray radiation field; and image output means for outputting the X-ray image data calculated by said second calculating means as a visible image.
 2. A system according to claim 1, wherein said X-ray mask means is an X-ray mask member comprising a plate of an X-ray transmitting material, on which a plurality of X-ray shielding segments are adhered.
 3. A system according to claim 2, wherein said X-ray shielding segments are lead segments.
 4. A system according to claim 1, wherein said drive means is means for automatically moving said X-ray mask means to be interlocked with said X-ray source and said X-ray image detection means.
 5. A system according to claim 1, wherein said X-ray image detection means includes X-ray/photo conversion means for converting X-ray data into visible image data, and camera means for converting an output image from said X-ray/photo conversion means into an electrical signal.
 6. A system according to claim 5, wherein said X-ray/photo conversion means is an image intensifier.
 7. A system according to claim 1, wherein said first calculating means calculates said scattered X-ray intensity distribution data with the use of an interpolation calculation technique utilizing a SINC function.
 8. A system according to claim 1, wherein said first calculating means has means for identifying whether or not present transmission X-ray intensity data masked by said X-ray mask means is said scattered x-ray intensity data, in accordance with whether or not the intensity data exceeds a predetermined threshold level.
 9. A system according to claim 1, wherein said storage means is a means having a memory area for transmission X-ray data obtained when said X-ray mask means is located in said X-ray radiation field which memory area is smaller per one frame than a memory area for transmission X-ray data which is obtained when said X-ray mask means is located outside the X-ray radiation field. 